1. Field of the Invention
This invention relates generally to communication networks. More specifically, this invention relates to performance monitoring in high-speed packet networks and time-division multiplexed networks.
2. Description of the Related Art
A number of acronyms well known in the art are summarized in Table 1.
TABLE 1ACRONYMMeaningAISalarm indication signalAPSautomatic protection switchingBERbit error rateBIPbit interleaved parityCVcode violationDS-Ndigital signal at level NESerrored secondsES-LFEfar end line errored secondES-SSection Errored SecondLOFloss of framingLOSloss of signalMPLSmultiple protocol label switchingOC-Noptical carrier level at level NOSoperations systemPDHplesiochronous digital hierarchyPOHpayload overheadRDIremote defect indicationREIremote error indicationSDHsynchronous digital hierarchySEFseverely eroded framingSEFseverely errored framingSESseverely errored secondSNMPsimple network management protocolSONETsynchronous optical networkSPEsynchronous payload envelopeSTMsignaling traffic managementSTS-Nsynchronous transport signals atlevel NTDMtime-division multiplexedTOHtransport overheadUASUnavailable seconds. A count of theseconds during which a layer wasconsidered to be unavailable.
High-speed communications networks continue to increase in importance in modern telecommunications. Efficient performance monitoring has become desirable in many kinds of networks, including optical networks, conventional data networks such as Ethernet, and MPLS ATM networks.
As an example, the Synchronous Optical Network (SONET) is a set of standards that define a hierarchical set of transmission rates and transmission formats for carrying high-speed, time-domain-multiplexed (TDM) digital signals. SONET lines commonly serve as trunks for carrying traffic between circuits of the plesiochronous digital hierarchy (PDH) used in circuit-switched communication networks. SONET standards of relevance to the present patent application are described, for example, in the document Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria (Telcordia Technologies, Piscataway, N.J., publication GR-253-CORE, September, 2000). While the SONET standards have been adopted in North America, a parallel set of standards, known as Synchronous Digital Hierarchy (SDH), has been promulgated by the International Telecommunications Union (ITU), and is widely used in Europe. From the point of view of the present invention, these alternative standards are functionally interchangeable.
There are four optical interface layers in SONET: path layer, line layer, section layer and photonic layer. These optical interface layers have a hierarchical relationship, with each layer building on the services provided by the lower layers. Each layer communicates with peer equipment in the same layer and processes information and passes it up and down to the next layer by mapping the information into a differently organized format and by adding overhead. In a simplified example, network nodes exchange information as digital signals (DS-1 signals) having a relatively small payload. At a source node of the path layer several DS-1 signals are packaged to form a synchronous payload envelope (SPE) composed of synchronous transport signals (STS) at level 1 (STS-1) along with added path overhead. The SPE is handed over to the line layer. The line layer concatenates multiple SPEs, and adds line overhead. This combination is then passed to the section layer. The section layer performs framing, scrambling, and addition of section overhead to form STS-Nc modules. Finally the photonic layer converts the electrical STS-Nc modules to optical signal and transmits them to a distant peer node as optical carriers (OC-N signals).
At the distant peer node, the process is reversed. First, at the photonic layer the optical signal is converted to an electrical signal, which is progressively handed over to lower levels, respective overheads being stripped off, until the path layer is reached. The DS-1 signals are unpackaged, and terminate at the destination node.
The lowest-rate link in the SONET hierarchy is the optical carrier level (OC-1) at the path layer, which is capable of carrying 8000 STS-1 frames per second, at a line rate of 51.840 Mbps. An STS-1 frame contains 810 bytes of data, which are conventionally organized as a block of nine rows by 90 columns. The first three columns hold transport overhead (TOH), while the remaining 87 columns carry the information payload, referred to as the synchronous payload envelope (SPE). The SPE contains one column of payload overhead (POH) information, followed by 86 columns of user data. The POH can begin at any byte position within the SPE capacity of the payload portion of the STS-1 frame. As a result, the SPE typically overlaps from one frame to the next. The TOH of each frame contains three pointer bytes (H1, H2, H3), which are used to indicate where in each frame the POH begins and to compensate for timing variations between the user input lines and the SONET line on which the STS-1 frames are transmitted.
STS-1 frames can efficiently transport DS-3 level signals, operating at 44.736 Mbps. The STS-1 frames themselves are not too much larger than DS-3 frames. When signals at rates below DS-3 are to be carried over SONET, the SPE of the STS-1 frame is divided into sections, known as virtual tributaries (VTs), each carrying its own sub-rate payload. The component low-rate signals are mapped to respective VTs, so that each STS-1 frame can aggregate sub-rate payloads from multiple low-rate links. Multiple STS-1 frames can be multiplexed (together with STS-Mc frames) into STS-N frames, for transmission on OC-N links at rates that are multiples of the basic 51.840 Mbps STS-1 rate.
Maintenance criteria are extensively specified in the above-noted Telcordia publication GR-253-CORE to enable the maintenance of the integrity of the network and individual network elements. Maintenance includes the general undertakings of (1) defect detection and the declaration of failures, (2) verification of the continued existence of a problem, (3) sectionalization of a verified problem, (4) isolation, and (5) restoration.
Performance monitoring, to which this application particularly relates, is important and sometimes essential to the conduct of the various above-mentioned tasks in network maintenance for data networks in general. Performance monitoring, as used herein, relates to in-service, non-intrusive monitoring of transmission quality. Network elements are required to support performance monitoring as appropriate to the functions provided at their respective levels in the network. Network elements are also required to perform self-inventory, by which a network element reports information to the performance monitor about its own equipment, as well as adjacency information concerning other network elements to which it is physically or logically connected. The above-noted Telcordia publication GR-253-CORE contains generic performance monitoring strategies, discusses various types of performance monitor registers (e.g., current period, previous period, and threshold registers), and defines performance monitor parameters for the various signals which are found in SONET communication.
A principal approach taken in SONET performance monitoring is the accumulation by network elements of various performance monitor parameters based on performance “primitives” that it detects in the incoming digital bit stream. Primitives can be either anomalies or defects. An anomaly is defined to be a discrepancy between the actual and desired characteristics of an item. A defect is defined to be a limited interruption in the ability of an item to perform a required function. The persistence of a defect results in a failure, which is defined to be the termination of the ability of an item to perform a required function. A large number of defects and failures are defined in the above-noted Telcordia publication GR-253-CORE.
Functionally, performance monitoring is performed at each layer, independent of the other layers. However, part of the functional model assumes that layers pass maintenance signals to higher layers. For example, a defect, such as Loss of Signal (LOS) occurring at the section layer causes an alarm indication signal (AIS-L) to be passed to the line layer, which in turn causes an alarm signal (AIS-P) to be transmitted to the STS Path layer. Thus, an AIS defect can be detected at a particular layer either by receiving the appropriate AIS on the incoming signal, or by receiving it from a lower layer. In consequence, performance monitor parameters at a level are influenced by defects and failures occurring at other levels.
Thresholds are defined for most of the performance monitor parameters supported by SONET network elements. These are used by the performance monitor to detect when transmission degradations have reached unacceptable levels. It is common for hysteresis to be employed before a declared defect or failure can be terminated, in order to assure stability of the system. Thresholds are widely used in the SONET protocol. For example, one type of threshold specifies when a defect should be reclassified as a failure. Another use is alarm generation when a performance monitor counter exceeds a predefined threshold.
Accumulation intervals are defined for each performance monitor parameter. Data accumulated in successive accumulation intervals are required to be independently maintained in a memory as a pushdown stack during a current day's operation. Each network element reports its statuses and results periodically to a higher authority or performance monitor management system. It is the responsibility of the performance monitor management system to derive time-based calculations such as the time during which a defect or failure persisted (errored seconds) and other performance monitor related parameters. Each of the parameters that have to be calculated is dependent on one or more variables related to SONET defects, SONET counters, and SONET failures.
For example, severely eroded seconds at the line level are monitored using the performance monitor parameter SES-L. This parameter is advanced if any of the following SONET defects was active during the previous second: severely eroded framing (SEF), loss of signal (LOS), and alarm indication signal (AIS-L).
As a second example, the counter CV-L counts coding violations at the line level. The performance monitor parameter SES-L is advanced if the SONET counter CV-L is above 9834.
Various linear automatic protection-switching architectures are commonly implemented in optical networks and many other networks, e.g., Ethernet, and MPLS, in order to provide fault tolerance. Examples of these are known as the 1+1 architecture, the 1:1 architecture and the 1:n architecture.
In the 1+1 architecture, the head-end signal is continuously communicated to both working and protection equipment, so that the same payloads are transmitted identically to the tail-end working and protection equipment. At the tail end, working and protection OC-N signals are monitored independently and identically for failures. The receiving equipment chooses either the working or the protection signal as the one from which to select the traffic. Because of the continuous head-end bridge, the 1+1 architecture does not allow an unprotected extra traffic channel to be provided.
In the 1:n architecture, there are n working channels, any of which can be bridged to a single protection line. Head-end to tail-end signaling is accomplished by using the secondary, or protection channel. Because the head end is switchable, the protection line can be used to carry an extra traffic channel.
The 1:1 architecture is actually a special case of the 1:n architecture, in which n is 1. It is specially mentioned mainly because there are conventions according to the above-noted Telcordia publication GR-253-CORE, which allow line terminating equipment employing the 1+1 architecture to interoperate with line terminating equipment employing the 1:1 architecture. These conventions are outside the scope of this disclosure.
When a user leases a protected network connection from a service provider, a certain quality-of-service level (QoS) is warranted. For example, the service provider may guarantee that over any X-minute period, the average bit rate of the connection will be no less than Y bps. To protect network integrity, the actual connection is typically made up of two or more physical lines, known as the working and protection lines. This protection arrangement is transparent to the user.
Conventionally, the operator has been able to obtain performance data on each of the working and protection lines individually. However, the network has not been set up to provide a collective reading for the protection pair. The performance of the lines the working and protection lines is monitored individually, often by separate processors, and without mutual coordination. Furthermore, the performance data are gathered at long intervals. Thus, the user and service provider have no straightforward way of checking the combined performance of the working and protection lines minute-by-minute. Such combined monitoring would be desirable in order to ensure that the service provider has met his QoS obligation even during time intervals in which protection switching occurs between the lines.